Progressive neurodegenerative disorders (PNDs), exemplified by Alzheimer's disease, cause a slow but inexorable loss of neurons that is accompanied by degrading cognitive or motor function and is followed by death of the afflicted individual. The effects of PNDs are devastating to the quality of life of those afflicted as well as that of their families. Moreover, PNDs impose an enormous health care burden on society. Indeed, as this class of diseases primarily affects the expanding elderly population, their prevalence and societal impact are expected to become even more severe in the coming years.
One of the most promising therapeutic approaches for treating PNDs is neuronal replacement with transplanted neurons derived from stem cells, which are found scattered throughout various tissues of the adult human body in very small numbers. Human embryonic stem cells (HESCs) are the most well characterized for potential therapeutic applications. Unfortunately, the development of HESC lines in sufficient quantity and of adequate quality for clinical applications has been severely hampered by controversy over their embryonic origin. However, even if clinical-grade HESC lines do become readily available, transplanting in vitro-differentiated, HESC-derived neurons is risky and requires highly invasive intracerebral injection of the neurons into a patient. Thus, there is an urgent and ongoing need for methods that afford low risk, non-invasive replenishment of neurons for treating PNDs or inhibiting their onset.